Interleukin-6 messenger RNA expression and interleukin-6 protein secretion in cells isolated from normal human bone: regulation by interleukin-1.

Recent evidence suggests that cytokines, in addition to regulating hematopoiesis and immune functions, may be important paracrine regulators of bone turnover. Interleukin-1 (IL-1) and IL-6 are cytokines that are produced by and affect both hematopoietic and nonhematopoietic cell types. IL-1 stimulates bone resorption and inhibits osteoblast proliferation and collagen production. Previous reports that IL-6 was secreted in murine osteoblast and bone organ cultures in response to IL-1 and PTH suggested that IL-6 has paracrine effects on bone resorption or formation. To determine whether IL-6 has a paracrine function in human bone, IL-6 expression in cells isolated from normal human bone was investigated. IL-6 mRNA levels in untreated cultures were low and variable, and IL-6 secretion was undetectable. PTH had no effect on IL-6 mRNA levels or IL-6 secretion. IL-1 beta increased IL-6 mRNA levels, maximally 40-fold at 12 h. IL-1 beta increased IL-6 secretion to 0.13 nM, more than 80-fold that of untreated controls at 12 h. IL-1 beta also increased IL-1 beta mRNA levels, maximally 9-fold at 12 h, but did not increase cellular levels or secretion of IL-1 beta protein. Recombinant human IL-6 at 0.5-5 nM stimulated resorption in neonatal mouse calvarial organ cultures but had no effect on human bone-derived cell DNA synthesis or type I procollagen mRNA levels. The results suggest that IL-6 production by human osteoblasts may function to enhance osteolytic activity of IL-1 but does not affect proliferative and matrix biosynthetic aspects of bone formation that were tested. Because osteoblasts and bone marrow cells are in close proximity, IL-6 produced by osteoblasts may also function to amplify IL-1 stimulation of immune responses and hematopoiesis in bone marrow.